This invention relates to a method of making tubular products, especially catheters. More particularly, this invention relates to a method for making a reinforced catheter having an irregular lumen surface to reduce friction when used as a guiding catheter for the passage of another catheter through the lumen or the rotation of another catheter within the lumen. Still more particularly, this invention relates to a method of making a catheter wherein either or both elongated axial surfaces of a catheter are modified to decrease mechanical friction due to fluid coupling by causing the surfaces to have a non-linear or non-smooth character, thus to limit contact area with any adjacent structure to relatively small areas or line or points rather than the entire geometric surface or relatively large surfaces.
As explained in U.S. Pat. No. 4,764,324, the art of manufacturing tubes, pipes or cannulae by extruding a plastic material to produce significant quantities of tubing is fairly well developed. In many instances, it is desirable to use reinforcement in the tubes or pipes to increase the pressure, tensioning, or torque-carrying capacities of those tubes or pipes. Ordinary plastic garden hose reinforced with filament is a common example of such a product made according to prior art techniques, as is a catheter for applications in the medical field.
Catheters of the type contemplated are relatively thin and flexible tubes which include inner and outer plastic layers with a wire sheathing embedded between the layers wherein the wire sheathing is either braided or cross-wound to obtain maximum torsional rigidity and a satisfactory longitudinal flexibility. A convention prior art process for making a reinforced extruded catheter is a three-step process. In the first step, a mandrel having an outside diameter about equal to the desired inside diameter of the finished catheter is passed through suitable extrusion tooling to cause a tubular jacket or sheath of the catheter material to form around the mandrel. In this step, the outside diameter of the first extrusion layer on the mandrel is smaller than the desired finished outside diameter of the finished catheter. Next, the inner core tube formed in the first step as described above is processed by suitable machinery to cause a pattern of reinforcing materials, such as wires, fibers, or monofilaments, for example, to be laid along and/or around or partially into and in contact with the surface of the core tube. Next, the composite intermediate structure of the inner core tube and the reinforcing layer thus applied is again passed through suitable extrusion tooling equipment to deposit a second layer of catheter material around and bonded to the composite thereby encapsulating the now reinforced inner core tube forming essentially a single structure. The outside diameter of the second layer of extrusion is approximately equal to the desired finished outside diameter of the catheter. Subsequently, finishing and polishing operations can be performed and a composite thus constructed cut to its desired length. The mandrel, if any, is then extracted by lengthwise pulling, leaving the hollow catheter tubing with reinforced walls. That process produces a catheter with smooth, uninterrupted inner and outer circumferential surfaces.
U.S. Pat. No. 4,764,324 to the applicant herein constitutes a significant improvement on that three-step process by recognizing that heating the polymer substrate, or the reinforcing material and a polymer substrate beneath it, during a process of manufacturing a catheter, while simultaneously applying axial tension to the reinforcement, will cause the reinforcement material to deform or penetrate the original surface of the catheter body polymer and thus penetrate into such a surface. The distance to which the reinforcement material sinks into the underlying polymer is highly controllable and repeatable depending on the conditions of the relative temperatures of the catheter body and the reinforcement material, as well as on the tension exerted on the reinforcing material, and the physical characteristics of the polymer from which the catheter is made. Thus, by controlling these parameters, the radial position of the reinforcement in the wall of the catheter can be simply but accurately followed. In the '324 patent, it had been noted that the polymer thus softened and deformed or penetrated but remaining outside of the new smaller diameter of the reinforcement structure produces a somewhat peaked or waffled contour in those locations where the polymer has exuded between the strands or filaments of the reinforcement and extends radially outwardly beyond the reinforcement to an extent where it can be worked to form the outer wall of the catheter body. However, the art suggested that smooth-walled catheters were desirable and users demanded such devices.
Accordingly, catheters made by either of the two processes noted above produced a catheter witch a smooth inner diameter or bore well, or lumen, as well as a smooth, outer diameter wall to form a smooth surface composite construction of polymer containing the imbedded reinforcement between the now smooth inner and outer surfaces.
In fact, for most reinforced medical tubing structures, placement of the reinforcement strand is desired at or near the mid-wall point of the structure. In the '324 patent, the depth of radial placement of the reinforcement strand is chiefly controlled by the degree of heat softening of the substrate at the time the reinforcement impinges the surface under essentially constant application tension.
Among medical tube structures are those that are used as mechanical guides or sheaths. One such tube is the guide for balloon catheters in the practice of angioplasty. In this use, since a balloon-carrying catheter is to slide through the previously-placed guiding catheter, it is desired to reduce internal friction between the guide tube and the balloon catheter to a minimum.
A smooth-walled guiding catheter, even one with a lining of a low friction material, such as a Teflon brand material, can exhibit a considerable drag friction due to fluid coupling where a think film of fluid "locks" the surface of the balloon to the catheter wall. An example of this phenomenon when air is the fluid can be found when meshing precision gauge blocks in a machine shop which, if pushed together tightly, will stick together even though there are no forces such as magnetism present. This occurs because the joint line between the blocks is so small that it does not readily allow air to enter the blocks to cause them to separate. Similarly, in the case of smooth-walled catheters and balloons, rubbing against the smooth guide tube wall displaces all but essentially a monolayer of the fluid present over a relatively large contact area, A=d1 where d is the diameter of the lumen or I.D. of the tube; and 1 is the length of contact, allowing forces of molecular friction to create an unacceptably high drag friction.
Accordingly, it is a problem in the art, even in the use of standard smooth-bore catheters, and even when stiffness and torque properties are excellent and friction is low when dry, that high levels of fluid friction are experienced as soon as blood or injectate are present to make the fluid friction with the tube unacceptable.
Accordingly, it is an overall problem addressed by this invention to develop a catheter and guide-tube pair which exhibit a marked reduction in axial and rotational drag forces during manipulation due to virtually complete elimination of viscous coupling between the adjacent surfaces.
Accordingly, it is an object of this invention to provide a catheter having an inner bore or lumen with an irregular surface to reduce contact friction when in use.
It is another overall object of this invention to provide a method and apparatus wherein either or both surfaces of a tubular object or the outer surface of a solid object are modified in such a way as to decrease mechanical friction due to fluid coupling by causing the surfaces to have a non-lineal or non-smooth character, thus limiting contact area within the adjacent structure to small areas of points or lines rather than the entire geometric surface.
It is another overall object of this invention to provide a method and apparatus for producing a catheter with modified and controlled surface geometry by processes of embossing patterns on the outside surface and molding patterns into tubular inner surfaces.
It is still another object of this invention to provide a tubular catheter having a modified lumen surface wherein a reinforcing braid is submerged in a layer at the time of pattern of generation by use of a correctly sized die heated to a point where it heats and pushes the reinforcement (or pattern) into a substrate to cause waffle-like distortion of an outer wall surface of the structure.
These and other objectives of the invention will become apparent from the drawings and the detailed description of the invention which follows.